1. Field of the Invention
The present invention relates to a gas turbine combustor and, more particularly, to a premix type of gas turbine combustor.
2. Description of the Related Art
As disclosed in, for example, Japanese Patent Unexamined Publication No. 56-25622, conventional combustors of this type which have generally been used are commonly provided with several stages of combustion sections and are arranged as a premix-combustion system so as to suppress the generation of NOx by combustion with a lean mixture.
FIG. 11 shows in cross section the essential portion of a typical combustor of this type. This combustor comprises a first-stage burner a (a plurality of diffusion burners for separately supplying fuel and air) disposed upstream of the combustor and asecondstage burner b (a similar diffusion burner) disposed downstream of the first-stage burner a in such a manner as to project into the combustor. The combustion chambers for the respective burners are divided by a throat portion into an upstream first-stage combustion chamber 1 and a downstream second-stage combustion chamber 2, with the throat portion having a diameter reduced compared to the line size and being formed between the combustion chambers 1 and 2.
The above-described combustor operates as follows. At the time of starting, fuel is first supplied to the first-stage combustion chamber 1 alone to fire the first-stage burner a. Then, fuel is supplied to the second-stage burner b to fire the second-stage burner b. In this state, both the first-stage burner a and the second-stage burner b bring about diffusion combustion.
Subsequently, the supply of fuel to the firststage burner a is stopped and the rate of fuel supplied to the second-stage burner b is increased by a corresponding amount, thereby extinguishing the first-stage burner a. At the same time, the amount of combustion at the second-stage burner b is increased.
Thereafter, by again supplying fuel to the first-stage burner a, the combustion chamber 1 for the first-stage burner a serves as a premixing chamber for merely premixing fuel and air, and premix combustion is effected in the second-stage combustion chamber 2. In other words, the steady running of the combustor is performed in the above-described state.
In the combustor which is arranged in the above-described manner, during the steady running, it is possible to extremely effectively realize low-NOx combustion since premix combustion is performed during the steady running. However, as described previously, during starting, i.e. during the change from diffusion combustion to premix combustion, since it is necessary to input fuel to the second-stage burner b at a high flow rate, the second-stage burner b is overloaded and the metal temperature of the combustor rises to an extremely high degree. In addition, prior to the change to premix combustion, the first-stage and second-stage burners a and b are both in the state of diffusion combustion and, therefore, a large amount of NOx is produced during this time, and, during the steady running, the second-stage burner burning in the state of diffusion combustion, which produces a relative larger amount of NOx than premixed combustion of the first-stage burner.